The Fischer-Tropsch process is often used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas, coal-bed methane, residual (crude) oil fractions and/or coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas). The synthesis gas is then converted in a second step over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated ebullated bed reactors. A suitable Fischer-Tropsch reactor has been described in U.S. Pat. No. 5,517,473. However, the reactor described in this reference describes a large, completely fixed cooling system, which makes manufacturing, transport and repair (e.g. in the case of a leakage) difficult.
The Fischer-Tropsch reaction is very exothermic and temperature sensitive with the result that careful temperature control is required to maintain optimum operation conditions and desired hydrocarbon product selectivity. Bearing in mind the very high heat of reaction which characterises the Fischer-Tropsch reaction the heat transfer characteristics and cooling mechanisms of a reactor are very important.
The heat transfer performance of fixed-bed reactors is limited because of the relatively low mass velocity, small particle size and low thermal capacity of fluids. If one attempts, however, to improve the heat transfer by increasing the gas velocity, a higher CO conversion can be obtained, but there is an excessive pressure drop across the reactor, which limits commercial viability. Increasing reactor capacity by increasing gas throughput and CO conversation may result in increasing radial temperature gradients. For thermal stability and efficient heat removal the Fischer-Tropsch fixed-bed reactor tubes should have a diameter of less than 5 or 7 cm. The desired use of high-activity catalysts in Fischer-Tropsch fixed-bed reactors makes the situation even worse. The poor heat transfer characteristics make local runaways (hotspots) possible, which may result in local deactivation of the catalyst. In order to avoid runaway reaction the maximum temperature within the reactor must be limited. However, the presence of temperature gradients within the reaction mixture means that much of the catalyst is operating at sub-optimal levels.
The use of liquid recycle as a means of improving the overall performance in a fixed-bed design has been described. Such a system is also called a “trickle bed” reactor (as part of a sub set of fixed-bed reactor systems) in which both reactant gas and liquid are introduced (preferably in a down flow orientation with respect to the catalyst) simultaneously. The presence of the flowing reactant gas and liquid improves heat removal and heat control thus enhancing the reactor performance with respect to CO conversion and product selectivity. A limitation of the trickle bed system (as well as of any fixed-bed design) is the pressure drop associated with operating at high mass velocities. The gas-filled voidage in fixed-beds (typically less than 0.50) and size and shape of the catalyst particles does not permit high mass velocities without excessive pressure drops. Consequently, the mass throughput undergoing conversion per unit reactor volume is limited due to the heat transfer rates. Increasing catalyst particle size and higher mass flow rates improve heat transfer (for a given pressure drop) and enable increased conversion capacity. However, the loss of catalyst selectivity and lower catalyst efficiency may offset the improved conversion capacity.
Three-phase slurry bubble column reactors generally offer advantages over the fixed-bed design in terms of heat transfer characteristics. Such reactors typically incorporate small catalyst particles suspended by upward flowing gas in a liquid continuous matrix. A plurality of cooling tubes are present in three phase slurry system. The motion of the continuous liquid matrix allows sufficient heat transfer to achieve a high commercial productivity. The catalyst particles are moving within a liquid continuous phase, resulting in, efficient transfer of heat generated from catalyst particles to the cooling surfaces, while the large liquid inventory in the reactor provides a high thermal inertia, which helps prevent rapid temperature increases that can lead to thermal runaway. An extensive description of three phase slurry bubble column reactors is given in W.-D. Deckwer, Bubble Column Reactors (John Wiley & Sons, Chichester, 1991).
Commercial fixed-bed and three-phase slurry reactors typically utilise boiling water to remove the heat of reaction. In the fixed-bed design, individual reactor tubes are located within a shell containing water/steam typically fed via flanges in the shell wall. The heat of reaction raises the temperature of the catalyst bed within each tube. This thermal energy is transferred to the tube wall forcing the water in the surrounding jacket to boil. In the slurry design, cooling tubes are placed within the slurry volume and heat is transferred from the liquid continuous matrix to the tube walls. The production of steam within the tubes provides the needed cooling. The steam in turn is used for heating purposes or to drive a steam turbine.
Synthesis gas leaking into the cooling system (identifiable through analysis of the steam output) cannot be separated, represents a risk and will force shutdown and repair of the slurry reactor. In light of the exothermic nature of the reaction and the typical volume of slurry reactors the shutdown process is both time consuming and expensive in terms of loss of production capacity. Where a known reactor utilises a single header with multiple interconnected tubes the identification and repair of a leaking tube is difficult. In light of these difficulties it is known to block off a leaking tube rather than to attempt repair. However, blocking off a relatively large number of leaking tubes has the disadvantage of reducing cooling capacity resulting in a part of the reactor being uncooled, or under cooled, with the possible formation of hotspots. In addition, the cooling capacity of the reactor decreases, resulting in a reactor which is loosing its intrinsical safety.
Another drawback of known slurry reactors is that the cooling tubes are fixed in place inside the reactor during construction. Typically the cooling tubes are welded to headers through which the tubes are fed with coolant. Such an arrangement involves dangers for personnel during inspection and repair of individual cooling tubes when the reactor is configurated for use. Furthermore, given their large size, commercial reactors generally have to be transported in a horizontal position. This leads to difficulties in ensuring that tubes within the reactors are not damaged or dislodged.